Use lispy tuples in cat (fixes JuliaLang#21673) (JuliaLang#39314)

The `cat` pipeline has long had poor inferrability.
Together with JuliaLang#39292 and JuliaLang#39294, this should basically put an
end to that problem.

Together, at least in simple cases these make the performance
of `cat` essentially equivalent to the manual version.
In other words, the `test1` and `test2` of JuliaLang#21673 benchmark
very similarly.

base/abstractarray.jl

@@ -1645,28 +1645,29 @@ _cat(dims, X...) = cat_t(promote_eltypeof(X...), X...; dims=dims)
     return __cat(A, shape, catdims, X...)
 end
 
-function __cat(A, shape::NTuple{M}, catdims, X...) where M
-    N = M::Int
-    offsets = zeros(Int, N)
-    inds = Vector{UnitRange{Int}}(undef, N)
-    concat = copyto!(zeros(Bool, N), catdims)
-    for x in X
-        for i = 1:N
-            if concat[i]
-                inds[i] = offsets[i] .+ cat_indices(x, i)
-                offsets[i] += cat_size(x, i)
-            else
-                inds[i] = 1:shape[i]
-            end
-        end
-        I::NTuple{N, UnitRange{Int}} = (inds...,)
-        if x isa AbstractArray
-            A[I...] = x
-        else
-            fill!(view(A, I...), x)
-        end
+# Why isn't this called `__cat!`?
+__cat(A, shape, catdims, X...) = __cat_offset!(A, shape, catdims, ntuple(zero, length(shape)), X...)
+
+function __cat_offset!(A, shape, catdims, offsets, x, X...)
+    # splitting the "work" on x from X... may reduce latency (fewer costly specializations)
+    newoffsets = __cat_offset1!(A, shape, catdims, offsets, x)
+    return __cat_offset!(A, shape, catdims, newoffsets, X...)
+end
+__cat_offset!(A, shape, catdims, offsets) = A
+
+function __cat_offset1!(A, shape, catdims, offsets, x)
+    inds = ntuple(length(offsets)) do i
+        (i <= length(catdims) && catdims[i]) ? offsets[i] .+ cat_indices(x, i) : 1:shape[i]
+    end
+    if x isa AbstractArray
+        A[inds...] = x
+    else
+        fill!(view(A, inds...), x)
+    end
+    newoffsets = ntuple(length(offsets)) do i
+        (i <= length(catdims) && catdims[i]) ? offsets[i] + cat_size(x, i) : offsets[i]
     end
-    return A
+    return newoffsets
 end
 
 """